Metal carbonyl anions

More remarkable than the formation of zero-oxidation-state metal carbonyls is the reduction of many of these carbonyl compounds to metal carbonyl anions, in which the metal has a negative oxidation state. The following example demonstrates that the two-electron reduction by sodium metal is accompanied by the loss of the two-electron donor carbonyl ligand, and so the 18-electron count on iron is preserved; the solvent is tetrahydrofuran (THF).

The stabilization of very electron-rich complexes, such as [Fe(CO)4]2−, is attributed to the back π bonding that shifts electron density from the metal to the carbonyl ligands, and this view is substantiated by C−O bond distances and other physical data. The metal atom in these carbonyl anions is assigned a negative oxidation state (−2 in the above example). This formalism does not acknowledge the delocalization of electron density from the metal to the ligand, but the chemical properties of the carbonyl anions do suggest that some of the negative charge resides on the metal. For example, a metal carbonyl anion can be protonated with the H+ ion, which generally attaches to the central metal and not a carbonyl ligand, as in the following example.[Fe(CO)4]2− + HCl → [HFe(CO)4]− + Cl−

Owing to their high reactivity, carbonyl anions are useful starting materials for the synthesis of other organometallic compounds, and this accounts for their applications in organic synthesis. For example, [Fe(CO)4]2− is used to extend a carbon chain by transfer of the carbonyl substituent, producing aldehydes, ketones, or carboxylic acids.

The variety of hydrocarbon ligands found in d-block organometallic chemistry range from simple σ-bonded alkyl ligands, double-bonded carbenes, and triply bonded carbynes to a host of polyene ligands, some of which are described in the remainder of this section.

Simple alkyl ligands

A simple alkyl ligand forms an M−C single bond, and in doing so the alkyl group acts as a one-electron monohapto ligand. Many such compounds are known, but they are less common in the d block than in the s and p blocks. This may in part be a result of the modest M−C bond strengths. Another reason for the limited stability of alkyl ligands in many d-block complexes is a set of reactions that can be quite rapid, such as β-hydrogen elimination, CO insertion, and reductive elimination. As described below, these reactions transform simple hydrocarbon ligands into other groups.

The β-hydrogen elimination reaction is an important feature of the hydrocarbon chemistry of both d- and p-block organometallics. The reaction consists of the abstraction of a hydrogen atom from the organic ligand with the formation of two products, one of which contains a metal-hydrogen bond and the other of which is an alkene.LnM−CH2CH3 → LnM−H + H2C=CH2 (Ln represents the n ligands not involved in the hydrogen elimination.) The reverse of this reaction, alkene insertion into the M−H bond, is illustrated by the hydroboration and hydrosilation reactions discussed above. Both the β-hydrogen elimination and addition of M−H across a C=C double bond are thought to proceed through a cyclic intermediate involving a three-centre, two-electron bond where a hydrogen atom bridges between the carbon and the metal atoms.

Since a compound with no hydrogen atoms on the β-carbon atom cannot undergo β-hydrogen elimination, benzyl, CH2(C6H5), and methyltrimethylsilyl, CH2Si(CH3)3 (shown below) ligands are generally more robust than ethyl ligands when attached to d-block metal atoms. Similarly, the lack of β-hydrogen atoms on the methyl group accounts for the greater stability of complexes containing the methyl ligand rather than the ethyl ligand.

A reaction frequently referred to as CO insertion leads to carbon-carbon bond formation between the carbon atom of a carbonyl ligand and the carbon atom of an alkyl ligand, which is the methyl group in the following example.

The CO insertion reaction is involved in all the transformations of [Fe(CO)4R]− into organic molecules.

Another type of reaction that can transform an attached organic ligand (as well as other groups) is reductive elimination.

The converse of reductive elimination is oxidative addition.

The reactions discussed above, insertion of C=C into an M−H bond, β-hydrogen elimination, and CO insertion, are often employed for the synthesis of organic molecules in the laboratory and in industry. They also account for some of the individual steps in some important catalytic cycles (see belowOrganometallic compounds in catalysis).

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